1. Field of the Invention
The present invention relates to an information recording and reproducing apparatus for recording and reproducing information to or from a multilayer optical information recording medium having a multilayer formation made of a plurality of information recording layers on which information is recorded.
2. Related Background Art
In recent years, mass-storage optical disks, typically CD or DVD, have come into wide use. Additionally, to further improve recording densities of optical disks according to demand for recording long-time video data or the like, a multilayer optical disk having a multilayer formation of two or more information recording layers is under development. In addition, if each information recording layer is formed by a phase change recording layer, it becomes possible to realize a multilayer optical disk capable of recording information in addition to reproducing information.
The phase change recording layer of the multilayer optical disk, however, is put in a crystalline state initially before recording, while it is put in an amorphous state after the recording. In general, an unrecorded area having the phase change recording layer in the crystalline state differs from a recorded area having the phase change recording layer in the amorphous state in a reflectance and a transmittance. Assuming such a condition that an information recording layer, which is an object of recording and reproducing, is irradiated with a laser beam in recording and reproducing into a recordable two-layer optical disk and that its reflected light is received, the operation of recording and reproducing on the information recording layer near the laser source is affected by a reflectance of an underlying information recording layer. Furthermore, during recording and reproducing to or from an information recording layer far from the laser beam emitting side, it is affected by a reflectance or a transmittance of an overlying information recording layer. Unless a uniform distribution is observed in the transmittance or the reflectance of each information recording layer, an intensity distribution may be uniformless in the optical beam of an optical detector. In this condition, the transmittance can be unified in a recorded area and an unrecorded area by appropriately adjusting an absorptance of a phase change recording material of the two-layer optical disk, while it is hard to cope with the uniformless reflectance.
Particularly, in performing a tracking servo in an information recording and reproducing apparatus for recording and reproducing to or from the multilayer optical disk, a variation of the reflectance remarkably affects the tracking servo. In other words, if the recorded area is mixed with the unrecorded area in the information recording layer, components caused by the variation of the reflectance are superposed on the light intensity distribution in the optical detector. Additionally, if a reflected light from the vicinity of a boundary portion between the recorded area and the unrecorded area is asymmetric with respect to a radial direction, it causes an offset in a tracking error on the information recording layer which is an object of recording and reproducing. The offset occurs in the tracking error since the effect of the boundary portion deflects the light intensity distribution in the disk radial direction even if a track is traced accurately in detecting a tracking error in the push-pull method generally used for the information recording and reproducing apparatus. As set forth hereinabove, there has been such a problem that a performance of the tracking servo is unsuccessfully secured due to the offset of the tracking error caused by a recording condition of each information recording layer when using a recordable multilayer optical disk.